The present invention relates to a process for making polyimides, and in particular to a process for making polyetherimides. The polyimides which are produced by the process of this invention are high performance engineering thermoplastics. These polymers are characterized by superior physical and chemical properties, such as high heat resistance, high impact and flexural strengths, and excellent processibility. Many of these polymers possess the physical characteristics of thermoset resins, yet can be molded conveniently by injection molding techniques.
Several processes for making polyimides are known. Typically, these polymers are prepared by reacting an organic diamine with an aromatic dianhydride. Two processes which have been of particular interest are the "melt polymerization" process and the "solution polymerization" process. The melt polymerization process has been described in several United States patents, representative of which is U.S. Pat. No. 3,803,085 by T. Takekoshi and J. Kochanowski. This process involves combining an aromatic bis(ether anhydride) and an organic diamine and heating the mixture under an inert atmosphere to form a homogeneous melt. Water formed during the polymerization reaction is removed at a temperature of up to 350.degree. C. In a preferred embodiment of the process, the final stage of the reaction is conducted under reduced pressure to facilitate removal of water. The melt polymerization procedure has been improved by employing certain catalysts to enhance yields or reaction rates. (For example, see Takekoshi et al., U.S. Pat. No. 3,833,544, Williams et al., U.S. Pat. No. 3,998,840 and Takekoshi, U.S. Pat. No. 4,324,822). The melt polymerization process has been adapted to the continuous mode by conducting the reaction in extrusion apparatus, as described, for example, by Takekoshi et al., U.S. Pat. No. 4,011,198 and Banucci, et al., U.S. Pat. No. 4,073,773.
Solution polymerization is generally conducted by reacting an aromatic dianhydride and an organic diamine in an inert solvent at temperatures up to about 200.degree. C. In this procedure, water evolved during the reaction may be removed by azeotropic distillation. The resulting polymer is generally recovered by mixing the reaction solution with a precipitant, such as methanol. The reaction solvents employed for solution polymerization are selected for their solvent properties and their compatibility with the reactants and products. High-boiling, non-polar, organic solvents are preferred. (E.g., see Takekoshi et al., U.S. Pat. No. 3,991,004.) Dipolar, aprotic solvents and phenolic solvents can also be used, particularly when an aromatic bis(ether dicarboxylic acid) is used in place of the dianhydride as the starting material. (E.g., see Takekoshi et al., U.S. Pat. No. 3,905,942.)
D. Heath and J. Wirth (U.S. Pat. No. 3,847,867) disclose a method for preparing polyetherimides which involves stirring a solution of an aromatic bis(ether anhydride) and an organic diamine in a dipolar, aprotic solvent under ambient conditions to produce a polyamide acid, casting the polyamide acid solution on a substrate to facilitate the removal of organic solvent, and then heating the substrate in a stepwise manner to 200.degree.-300.degree. C. to complete the conversion of the polyamide acid to the polyetherimide.
S. L. Parekh (U.S. Pat. No. 4,417,044) discloses a process for making polyetherimides which involves reacting an aromatic bis(ether anhydride) with an organic diamine in an inert solvent to form a prepolymer solution, forming a thin film of the prepolymer solution to evaporate the solvent, and then heating the prepolymer to a temperature above the glass transition temperature of the final product to form the desired polyetherimide.
The use of the foregoing procedures for the preparation of certain high performance polymers has sometimes met with certain disadvantages. There is an increasing interest in developing injection moldable polyimides suitable for very high-temperature applications and having increased chemical resistance, as compared to the polyetherimides described in the above-cited patents. In general, these polyimides are crystalline or semi-crystalline homopolymers or copolymers prepared from aromatic dianhydrides and diamines containing rigid, linear monocyclic or polycyclic aromatic groups.
These crystalline and semi-crystalline polymers are highly insoluble in organic solvents. Moreover, when they are produced by first isolating the polyamide acid intermediate, it has been found that a substantial amount of the unreacted diamine is bound to the polyamide acid through relatively labile ionic bonds. When these materials are extruded at elevated temperatures, the ionic bonds are brokent and a significant amount of the diamine is lost through volatilization. The volatilization not only makes controlling the stoichiometry difficult, but also poses a significant health hazard since the diamine is lost to the atmosphere and condenses on surrounding surfaces.
Takekoshi (U.S. Pat. No. 4,221,897) describes reacting an aromatic dianhydride and an organic diamine in an aqueous reaction medium substantially devoid of organic solvent. The reaction produces a polyamide acid intermediate which is recovered as a finely divided powder which can be used to make high molecular weight polyimide by melt extrusion. In a similar process, Banucci et al. describe, in U.S. Pat. Nos. 4,098,800 and 4,197,396, a process which involves reacting an aromatic dianhydride and an organic diamine in an inert liquid selected from methylene chloride, chloroform, 1,2-dichloroethane and mixtures thereof with acetone. The reaction produces an oligomeric polyamide acid which is substantially insoluble in the organic liquid and thus separates from the reaction mixture as a precipitate. The polyamide acid may be recovered in powdered form, which is useful in powder coating procedures wherein the desired polyetherimide is obtained in in situ by heating it to a temperature above the glass transition temperature.
The melt polymerization procedures are attended by the disadvantages of complexity of operation, relatively high equipment costs and thermal limitations. Accordingly, a need exists for a simple procedure for preparing polyimides, particularly crystalline and semi-crystalline polyimides, which can be conducted in low-cost conventional equipment.